Hmm yes, ascian manipualtion is also very likely. Especially with that scene at the end of 5.1. I just find it a bit odd that Emet-Selch made no mention of any and he seemed the only one. It's not as if he was inclined to be transparent about their plans, though...

There's also The Burn, that was done by Garleans, wasn't it? So perhaps it was a Garlean settlement that was destroyed by whatever is causing the same thing that causes them to not have magic.